The present invention generally relates to a contactless ignition apparatus for an internal combustion engine in which an ignition coil of resin molded closed magnetic circuit type ignition coil is employed. More particularly, the present invention is concerned with a structure for supressing or reducing noise generated by the ignition apparatus.
In the closed magnetic circuit (path) type ignition coil which is employed in the contactless ignition apparatus for the internal combustion engine, the magnetic circuit is implemented as the closed path by combining appropriately E- or L-shaped laminated cores each formed of a lamination of silicon steel plates or sheets. By virtue of this structure, the closed magnetic circuit type ignition coil and hence the contactless ignition apparatus can enjoy profitable features such as high efficiency, capability of miniaturizing the size of the ignition coil, improved insulation property and high vibration withstanding capability owing to the use of thermosetting resin such an epoxy resin or the like as the insulating material, and so forth. For this reason, the closed magnetic circuit type ignition coil is widely used in place of the oil-filled open magnetic circuit type ignition coil.
However, the contactless ignition apparatus in which the closed magnetic circuit type ignition coil is used has encountered a problem that upon supplying high energy to an ignition plug connected to a secondary winding of the ignition coil by interrupting a current flowing through a primary winding, noise is generated by the ignition coil itself.
The noise of concern is observed mainly in a low frequency band of several hundred kilohertzs or less, i.e. in the AM band of radio broadcasting and can not be suppressed sufficiently by the noise reducing measures adopted heretofore such as connection of a capacitor to the primary circuit of the ignition coil (as is disclosed, for example, in Japanese Utility Model Publication No. 6464/1988). As a result, reception of radio broadcast programs in the areas where the electric field is enfeebled or the reception in the unfavorable conditions such as experienced within a motor vehicle equipped with a receiving antenna incorporated integrally in a window glass undergoes disturbance more or less.
As the result of noise measurements performed by the inventor of the present application on a variety of ignition apparatuses in a frequency band of several hundred kilohertzs or less, it has been found that
(1) noise of significant magnitude is observed even in the case of distributorless ignition apparatuses which are generally known as the low-noise device, and
(2) in the case of the contactless ignition apparatuses,
(a) some of the oil-filled iron-case type ignition coils are relatively less liable to noise generation, while
(b) some of epoxy resin filled or mold type ignition coils generate remarkable noise while the others are less prone to generate noise.
FIGS. 1 and 2 of the accompanying drawings show schematically arrangements of testers employed in the measurements mentioned above. In the figures, a reference numeral 1 denotes a closed magnetic path type ignition coil having a primary winding 10 and a secondary winding 11. A numeral 2 denotes an ignition plug connected to the secondary winding 11. A numeral 3 denotes an igniter which performs ON/OFF or interruption control of an electric current flowing to the primary winding 10 for thereby generating sparks at a predetermined ignition timing. Further, reference numeral 4 denotes a battery, 5 denotes a current probe for detecting the current flowing to the primary winding 10, numeral 6 denotes an oscilloscope used for allowing the current detected by current probe to be visually observed or recorded. A numeral 22 denotes an electric field probe disposed at a position distanced from the ignition coil 1 by 10 cm for detecting the field strength (intensity) of noise emitted by the ignition coil 1. A numeral 20 denotes a current probe for detecting a high-frequency current flowing through a power supply line for the ignition coil 1. Finally, a reference numeral 21 denotes a field strength indicator to serve for indicating the strength of the electric field detected by the electric field probe 22 and the high-frequency current detected by the current probe 20.
A variety of ignition coils including the molded coils and the cased coils were tested as samples or specimens by measuring noise radiation and noise current (or current noise) emitted from the ignition coils through the method illustrated in FIG. 1, the results of which are summarized in the following table 1.
TABLE 1 __________________________________________________________________________ Measuring Frequency 300 kHz Field Strength Specimen Filler Core Specification (Radiation Noise No. Type Material Structure of Winding Noise) Current dI/dT __________________________________________________________________________ 1 Molded Epoxy Closed Specification A 100 54 0.86 Coil Resin Magnetic Path 2 .vertline. .vertline. .vertline. Specification B 92 50 0.42 3 Cased Oil Opened Specification C 72 42 0.20 Coil Magnetic (Iron Path Sheet Casing) 4 Cased .vertline. .vertline. .vertline. 96 44 0.20 Coil (Glass Casing) dB .mu.V/m dB .mu.A A/.mu.s __________________________________________________________________________
It is apparent from the table 1 that
(a) in the case of the molded type ignition coil, difference is found in the radiation of noise in dependence on the coil specifications (core structure and winding specifications), and
(b) in the case of the iron sheet case type ignition coil, the iron case or housing is effective as a shield for the noise radiation.
Subsequently, the ignition coil identified by the specimen No. 1 in the table 1 was modified by removing deliberately the secondary winding to prepare a specimen No. 5 which was then measured with regard to noise by the method similar to that illustrated in FIG. 1, the results of the measurement being listed in the following table 2.
TABLE 2 __________________________________________________________________________ Field Strength Specimen Filler Core Specification (Radiation Noise No. Type Material Structure of Winding Noise) Current __________________________________________________________________________ 1 Molded Epoxy Closed Specification A 100 54 Coil Resin Magnetic Path 5 .vertline. .vertline. .vertline. Primary Winding A 100 53 Without Secondary Winding dB .mu.V/m dB .mu.A __________________________________________________________________________
As can be seen from the table 2, remarkable noise in the frequency band of several hundred kilohertzs or less (at 300 kHz in the measurement actually performed) is radiated only by turning on and off the primary winding regardless of presence or absence of the secondary winding, i.e. notwithstanding of the absence of electric discharge at the ignition plug 2 shown in FIG. 1.
FIG. 3 of the accompanying drawings shows a waveform of the primary current of the ignition coil measured synchronously with that of the noise current at 300 kHz recorded by the method illustrated in FIG. 2. From these waveforms, it could be confirmed that noise of greater magnitude is generated at the time of interrupting the primary current of the ignition coil than at the time when the coil is turned on.
Under the circumstances, the primary current falling rate dI/dt (slope of the trailing edge of the primary current making appearance upon turning-off of the ignition coil) was checked by enlarging the waveform of the primary current at the turn-off time point for each of the ignition coils of various winding specifications listed in the Table 1 through the method similar to that shown in FIG. 2. As the result of this, it has been found that noise in the frequency band of several hundred kilohertzs or lower bears a relationship to the slope of the trailing or falling edge (hereinafter referred to as falling rate) of the primary current of the ignition coil.